Length of One Day
The length of one day (technically, a solar day) on a planet is the time from noon to noon. Since the planets spin on their axes at different rates and travel about the Sun at different speeds as well, the length of one day is different for each planet. For example, on Earth the length of a solar day is 24 hours. But because of its slow rotation and its rapid orbital speed, one solar day on Mercury lasts about as long as 176 days on Earth.
How long is a day on Mercury? Watch the Sun rise and set on Mercury in the "A Day on Mercury" animation. Click on the image above.